1997 good:
Inclusion of some Biggs footage.
Better music than Yub Nub to end the saga.
Restoring the Jabba scene in ANH was good in concept, but ...
1997 bad:
CG Jabba wasn't very good looking, and his dialogue was ultimately repetitive.
Luke's wussy scream in ESB
2004 Good:
CG Jabba was much better
getting rid of Luke's wussy scream
vastly improved Rancor compositing.
Ian McD playing the Emperor, but ...
2004 Bad:
Nonsensical dialogue added to Ian's scene in ESB. Couldn't just reshoot the same dialogue could they?
Bad title card in ANH
New Krayt call in ANH
Screwed up surround channels and missing Yavin fanfare in ANH
"Wesa Free" in ROTJ
Hayden Christensen ruining Anakin's redemption. ROTJ
Both versions bad:
Hokey robot/jawa slapstick in the Mos Eisley entrance
Greedo shooting first
Jedi Rocks